The subject matter disclosed herein relates to signal transmitting and receiving devices, and more particularly, to a seal assembly for an antenna and the method of making the same.
Devices for transmitting or receiving signals, such as antennas, are used in many diverse applications, including applications where the attenuation level of a signal is measured as between two antennas. For example, the attenuation of a radio frequency (“RF”) signal can be used to monitor certain performance characteristics of filters, such as diesel particulate filters (“DPF”).
A DPF is a device designed to trap and remove diesel particulate matter (i.e., soot) from the exhaust gas of diesel engines as the exhaust gas passes through the DPF in order to reduce emissions and improve efficiency. Since a DPF must periodically be cleaned when the soot loading of the DPF exceeds a certain threshold, a DPF monitoring system with DPF sensors can be employed to monitor the soot loading of a DPF. In a DPF monitoring system using RF signals, the power of an RF signal transmitted by an antenna located on one side of the DPF is compared to the power of that RF signal received by an antenna located on the other side of the DPF to measure the attenuation in the signal caused by the DPF. A DPF sensor or engine control module can then correlate the attenuation caused by the DPF with the amount of soot loading of the DPF. For example, a particular attenuation value caused by the DPF coupled with other data (e.g., temperature across the DPF) indicates a particular amount of soot loading of the DPF. Once the soot loading reaches a certain threshold as determined by the measured attenuation and other factors, the DPF must be cleaned or replaced.
Typically, these DPF monitoring systems are calibrated to account for noise and other system inconsistencies to manage the overall performance, reliability, and quality of the data collected, e.g., during operation of the DPF monitoring system. This calibration can take into account, for example, reflection of the RF signal that occurs as a result of an impedance mismatch between the coaxial cable and the antenna, which are each designed to have matching characteristic impedances of 50 ohms to minimize reflection of a portion of the signal back into the coaxial cable. Ideally, two antennas of the same construction and produced by the same manufacturing process would have the same characteristic impedance. But based on differences that result from the manufacturing process, antennas of the same construction often have varying characteristic impedances.
One source of the variability in characteristic impedance between antennas is the use of a slug (e.g., made of glass) to form a seal around the conductor element (i.e., the transmitting or receiving element), sealing the antenna body and forming an air gap around the conductor element. The configuration (e.g., shape and dimensions) of this air gap determines the characteristic impedance of the antenna. During manufacturing, the slug is melted and flows around the conductor element to form a seal. A portion of the seal material can also slump or flow into the air gap of the antenna body, which impacts the characteristic impedance and related reflectivity, of the antenna. For example, one antenna where the seal slumps further into the air gap than another antenna will have a different reflectivity than the other antenna. In existing antenna manufacturing processes, the distance that the seal slumps into the air gap varies from antenna to antenna, which results in significant variability between antennas.
Based on the differences in characteristic impedance between antennas, one antenna having a particular characteristic impedance might produce one attenuation reading while a replacement antenna having a different characteristic impedance might produce a different attenuation reading. Accordingly, when an existing antenna is replaced by a new antenna or when the existing antenna fails or requires maintenance, it is necessary to recalibrate the monitoring system since the characteristic impedance of the new antenna likely differs from the characteristic impedance of the existing antenna. This calibration takes time and resources, and often requires specific equipment and technical knowledge that are not necessarily available or cost effective to provide on-site. In addition, this variability in characteristic impedance can increase the amount of reflection of the signal caused by the impedance mismatch between the coaxial cable and the antenna. Reflection can disrupt the RF signal conduction and reduce the sensitivity of the antenna. Therefore, there is a need to reduce the variability between antennas, including the variability in reflectivity caused by variability in the depth that the seal slumps into the air gap of an antenna when forming a seal.
The discussion above is merely provided for general background information and is not intended to be used as an aid in determining the scope of the claimed subject matter.